Allow image uniforms to be used in separable programs This change updates mActiveImagesMask in ProgramExecutable for separable programs. Previously image uniforms would only work in the first shader stage. Bug: angleproject:4512 Tests: ProgramPipelineTest31.ImageUniforms Change-Id: Ib35bed94bb8fac883fd67f8bfa052186926d0ce4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3131582 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Brandon Schade <b.schade@samsung.com>